甘肃金川Ⅱ号岩体辉石化学特征及其地质意义

MINERAL CHEMISTRY AND GEOLOGICAL SIGNIFICANCE OF PYROXENE FROM SEGMENT Ⅱ OF THE JINCHUAN INTRUSION,GANSU PROVINCE

金川Ⅱ号岩体主要由二辉橄榄岩和硫化物橄榄岩构成，单斜辉石和斜方辉石为岩相中主要造岩矿物，单斜辉石含量一般高于斜方辉石。矿物间的结构关系表明主要造岩矿物的结晶顺序为：橄榄石-斜方辉石-单斜辉石-斜长石。单斜辉石的TiO2，Al2O3，Cr2O3和Na2O质量分数依次为：0．29％～1．26％、3．17％～3．59％、0．86％～1．14％、0．29％～0．57％，斜方辉石的TiO2，Al2O3，Cr2O3和Na2O质量分数普遍低于单斜辉石，依次为：0．16％～0.40％、1．85％～2．38％、0．48％～0.68％、0．03％～0．10％。元素间的相关关系显示辉石的类质同像置换方式为：^MIFe^2＋ ＋2Si=^MITi＋2^IVAl,^MIFe^2＋ ＋Si=^MIFe^3＋ ＋^IV Al,Fe^3＋和Ti进入矿物晶格置换Fe^2＋来平衡Al置换Si产生的正电价差。两种辉石较低的TiO2，Al2O3和Na2O质量分数显示出母岩浆为拉斑玄武岩浆的特征。二辉石温度计和单辉石温度计算表明斜方辉石的结晶温度下限和单斜辉石结晶温度上限大致在1146℃～1166℃之间，单斜辉石的结晶一直持续到1032℃或者更低。金川Ⅱ号岩体单斜辉石演化明显表现出向透辉石端员偏离，暗示岩浆在结晶分异过程中，可能与富钙质围岩发生了同化混染作用。

The Segment Ⅱ of the Jinchuan intrusion was composed mainly of lherzolite and sulfide dunite. Clinopyroxene and orthopyroxene were the main minerals for the two rock types, with the content of the former more than the latter one. The relationship of mineral texture indicated that the successive crystallization sequence of the Segment Ⅱ was olivine, orthopyroxene, clinoyroxene and plagioclase. Chemical analysis of cliopyroxene was characterized by TiO2 （0. 29%- 1.26% ）, Al2O3（3.17%-3.59%）, Cr203（0.86%-1.14%） and Na2O （0.29%-0.57%）, and orthopyroexen by TiO2 （0.16%-0.40%）, Al2O （1.85%-2.38%）, Cr2O3 （0.48%-0.68%） and Na2O （0.03%-0.10%）. The correlations of elements of pyroxene suggested that the elements substitution in pyroxenes were ： ^MIFe^2＋ ＋2Si=^MITi＋2^IVAl,^MIFe^2＋ ＋Si=^MIFe^3＋ ＋^IV Al. The octahedral bound Fe^3＋ and the octahedral Ti entered pyroxene crystal structure and replaced Fe^2＋ to balance the Al in the tetrahedral site and replaced the tetrahedral Si. The lower contents of TiO2, Al2O and Na2O of pyroxene demonstrated that the sources of magma of the intrusion were characteristic of basalt magma. Equilibrium temperature of pyroxene was calculated by two- pyroxene geothermometers （TBMSS and TBK90 ） and single-pyroxene geothermometer （TNT00Di）. The results showed that the lowest crystallization temperature of orthopyroxene and the highest of cli- nopyroxene were 1 146 ℃ to 1 166 ℃ and the crystallization temperature of clinopyroxene persisted to 1 032 ℃ or lower. The evolution of pyroxene from segment II of the Jinchuan intrusion leaned to diopside obviously,which imply that the magma assimilated carbonate-rich rock when it experienced fractional crystallization.